An ET (Envelope Tracking) system is known in which in order to increase the power efficiency of a High Power Amplifier (HPA) the power supply voltage (drain voltage) of the amplifier is modulated according to the envelope of an input signal (RF signal) to the amplifier (see, for example, Japanese Unexamined Patent Publication No. 2009-290283-A). By making the drain voltage variable, the amplifier can be allowed to operate in a high efficiency region.
In addition, as with the ET system, as techniques for increasing power efficiency using an envelope signal, an EER (envelope elimination and restoration) system, etc., are also known.